Programmable-resistance memory cell

ABSTRACT

The present invention relates to a memory cell comprising: a resistive structure; at least two electrodes coupled to the resistive structure, and at least one hydrogen reservoir structure, wherein the application of an electrical signal to one of the at least two electrodes causes the electrical resistance of the resistive structure to be modified by altering a hydrogen-ion concentration in the resistive structure.

FIELD OF THE INVENTION

The present invention relates to a programmable resistance memory cell,a method of fabrication therefor and a non-volatile memory deviceincorporating such a memory cell.

BACKGROUND OF THE INVENTION

For memory devices and for numerous other applications, bi-stabledevices or circuits are used. For example, for storing one bit ofinformation in a memory, a bi-stable device can be used which isswitchable between at least two different and persistent states. Whenwriting a logical “1” into the device, it is driven into one of the twopersistent states and when writing a logical “0”, or erasing the logical“1”, the device is driven into the other of the two different states.Each of the states persists until a next step of writing informationinto the device or erasing information in the device proceeds.

Flash erasable programmable read only memory (FEPROM, also referred toas flash memory) is used in semiconductor devices and provides for rapidblock erase operations. Typically, flash memory only uses one transistorper memory cell versus the two transistors per memory cell for knownelectrically erasable programmable read only memory (EEPROM). Thus,flash memory takes up less space on a semiconductor device and is lessexpensive to produce than EEPROM. Nevertheless, the development offurther space-saving components of semiconductor devices andcost-efficient fabrication techniques for producing such devicescontinues.

To that end, the use of materials with bi-stable electrical resistancefor semiconductor device applications has been studied. The resistancestates of the material can be changed reversibly by applying appropriateelectrical signals to the material. These electrical signals should belarger than a given threshold V_(T) and longer than a given time t. Theresistance state of the material can be read or analysed by applyingother signals which are non-destructive to the conductivity state ifthey are much smaller than V_(T).

Transition-metal oxides are one class of materials that can beconditioned such that they exhibit the desired bi-stable electricalresistance. Non-volatile two-terminal memory devices based ontransition-metal oxides have been disclosed. Such devices comprise atleast one memory cell, which comprises the arrangement of at least twoelectrodes being provided in contact with a transition-metal oxidelayer. Depending on the polarity of electrical pulses applied to one ofthe electrodes relative to the other electrode, the electricalresistance of the transition-metal oxide switches reversibly between atleast two different and persistent states. An example of such a deviceis given in U.S. Pat. No. 6,815,744.

The conditioning process that the transition-metal oxides are subjectedto in order that the switching between the resistance states may be donecomprises subjecting the transition-metal oxide to an appropriateelectrical signal for a sufficient period of time, this being done viaelectrical signals applied to the electrodes contacting thetransition-metal oxide layer as discussed above. The conditioningprocess generates a confined conductive region in the transition-metaloxide that can be reversibly switched between two or more resistancestates.

Drawbacks of the above-described devices are associated with theconditioning process. This is because, not only is the conditioningprocess time-consuming, it is required per cell incorporated in such adevice. Furthermore, the confined conductive region that is generated bythe conditioning process occurs at an arbitrary position in thedielectric material, i.e., the position of the conducting path is notcontrollable by well-defined process parameters. This may cause a largevariation in the electrical properties of such devices, that areotherwise nominally identical, to be observed. In combination, theseissues pose severe drawbacks for the use of memory cells based ontransition-metal oxides in a production-type array.

Accordingly, it is desirable to provide a programmable resistance memorycell that mitigates and/or obviates the drawbacks associated to knownprogrammable resistance memory cells.

SUMMARY OF THE INVENTION

According to an embodiment of a first aspect of the present invention,there is provided a memory cell comprising: a resistive structure; atleast two electrodes coupled to the resistive structure, and at leastone hydrogen reservoir structure, wherein the application of anelectrical signal to one of the at least two electrodes causes theelectrical resistance of the resistive structure to be modified byaltering a hydrogen-ion concentration in the resistive structure.

In order to initiate a conditioning process so that a confinedconductive region is formed in the resistive structure, an electricalsignal, such as, for example, an electrical pulse, is applied to one ofthe at least two electrodes relative to the other of the two electrodes.Application of the electrical signal also causes ionization of hydrogenthat has migrated from the hydrogen reservoir structure and into theresistive structure. The mobility of the hydrogen ions contributes tothe conditioning process being accelerated and the electrochemicalreactions associated with the conditioning process being of reducedduration compared to previously-proposed devices. Not only is the timetaken for the conditioning process reduced, but also some of thenon-uniformities associated therewith are reduced. Thus, memory cellshaving a lower statistical spread of operating characteristics and beingof increased reliability than previously-proposed memory cells may beproduced.

Preferably, at least one of the electrodes and/or the resistivestructure is at least partially embedded in the hydrogen reservoirstructure. In this way, the area over which migration of the hydrogenfrom the hydrogen reservoir structure into the resistive structure isincreased. Hence, this feature may contribute to further acceleratingthe conditioning process.

Desirably, the hydrogen reservoir structure is provided integrally to atleast one of the electrodes. The present feature gives the advantagethat the number of fabrication steps of producing an embodiment of thepresent invention may be reduced. Since the hydrogen reservoir structureis provided integrally to at least one of the electrodes rather than inthe dielectric material that is typically formed around the memory cell,such as, for example, silicon dioxide, a memory cell according to anembodiment of the present invention also gives the advantage of volumeefficiency when implemented in an array of memory cells since the spacebetween adjacent memory cells is not taken up by the hydrogen reservoirstructure, so allowing denser packing of the memory cells.

Preferably, at least one of the electrodes is permeable to hydrogen.This feature aids the migration of hydrogen from the hydrogen reservoirstructure into the resistive structure where it contributes toaccelerating the conditioning process.

Desirably, at least one of the electrodes comprises a coupling layer forcoupling the electrode to the resistive structure, the coupling layercomprising a material that absorbs at least 0.1 weight-percent ofhydrogen. Since the material of the coupling layer is selected to absorband/or have an affinity for hydrogen, the probability of the hydrogenmigrating from the hydrogen reservoir structure being drawn into theresistive structure, where it contributes to accelerating theconditioning process, is increased. In this regard, it is preferablethat the material of the coupling layer comprises one of: palladium(Pd), iridium (Ir), rhodium (Rd), a hydride and an alloy comprisinghydrogen. In this regard and for the sake of example, the hydride maycomprise one of yttrium hydride (YH₂) and lanthamum hydride (LaH₂) andthe alloy may comprise one of magnesium nickel hydride (Mg₂NiH₄) andlanthamum nickel hydride (LaNi₅H₆).

Desirably, the hydrogen reservoir structure comprises a first dielectricmaterial. This feature provides the advantage that, by being made up ofnon-conducting material, the presence of the hydrogen reservoirstructure does not interfere with the conductive region formed in theresistive structure during the conditioning process.

Preferably, the first dielectric material comprises one of: ozonetetraethoxysilane (TEOS), a transition-metal oxide, a metal hydroxideand a zeolite. In this regard and for the sake of example, thetransition-metal oxide may be hydrogen tungsten oxide (H-WO₃), the metalhydroxide may be one of aluminium hydroxide (Al(OH)₃), strontiumhydroxide (Sr(OH)₂) and calcium hydroxide (Ca(OH)₂) and the zeolite maybe hydrogen silicon oxide (H₂Si₂O₅).

Preferably, the resistive structure comprises a transition-metal oxide.The ionic mobility of hydrogen provides increased conductivity in theconductive region formed in the transition-metal oxide material and socontributes to accelerating the conditioning process.

According to an embodiment of a second aspect of the present invention,there is provided a non-volatile memory device comprising at least onememory cell according to an embodiment of the first aspect of thepresent invention. As discussed above, in an embodiment of the firstaspect of the present invention, not only is the time taken for theconditioning process reduced, but also some of the non-uniformitiesassociated with the conditioning process are reduced. Thus, memory cellshaving a lower statistical spread of operating characteristics and beingof increased reliability than previously-proposed memory cells may beproduced. Such properties are also displayed by a non-volatile memorydevice incorporating a memory cell according to an embodiment of thefirst aspect of the present invention.

Preferably, in an embodiment of the second aspect of the presentinvention, the hydrogen reservoir structure is provided so as to beshared by at least two memory cells. This feature provides the advantagethat the number of fabrication steps is reduced as are costs associatedwith the fabrication in that not as much material for the hydrogenreservoir structure is used compared to if it were provided per memorycell. A further advantage of the capability of denser packing of thememory cells is also provided by this feature.

Corresponding method aspects are also provided and thus according to anembodiment of a third aspect of the present invention, there is provideda method for fabricating a memory cell comprising the steps of:providing a resistive structure; coupling at least two electrodes to theresistive structure, and providing at least one hydrogen reservoirstructure.

Any of the device features may be applied to the method aspect of theinvention and vice versa. Features of one aspect of the invention may beapplied to another aspect of the invention. Any disclosed embodiment maybe combined with one or several of the other embodiments shown and/ordescribed. This is also possible for one or more features of theembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will now be made, by way of example, to the accompanyingdrawings, in which:

FIG. 1A schematically illustrates an embodiment of the presentinvention;

FIG. 1B schematically illustrates an alternative arrangement of theembodiment shown in FIG. 1A;

FIG. 2A schematically illustrates another embodiment of the presentinvention;

FIG. 2B schematically illustrates an alternative arrangement of theembodiment shown in FIG. 2A;

FIG. 3 shows experimental results pertaining to an embodiment of thepresent invention, and

FIG. 4 schematically illustrates a method according to an embodiment ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Within the description, the same reference numerals or signs are used todenote the same parts or the like.

FIG. 1A schematically illustrates a memory cell 10 according to anembodiment of the present invention. The memory cell 10 comprises aresistive structure 1. The resistive structure 1 is provided between atleast two electrodes M1, M2. The material of the resistive structure 1is chosen so that the resistive structure 1 exhibits at least two stableresistance states, it being possible to switch the electrical resistanceof the resistive structure 1 between the exhibited resistance states bythe application of an electrical signal to one of the electrodes M1, M2relative to the other of the electrodes M1, M2. The memory cell 10 alsocomprises at least one hydrogen reservoir structure 2.

Although an embodiment of the present invention will be describedhereinafter with reference to the resistive structure 1 being made of atransition-metal oxide, the present invention is not limited thereto andresistive structure 1 may be selected to comprise any other appropriatematerial.

In order to initiate a conditioning process so that a confinedconductive region is formed in the transition-metal oxide, an electricalsignal, such as, for example, an electrical pulse, is applied to one ofthe at least two electrodes M1, M2 relative to the other of the twoelectrodes M1, M2. Application of the electrical signal also causesionization of hydrogen that has migrated from the hydrogen reservoirstructure 2 and into the transition-metal oxide of the resistivestructure 1. The mobility of the hydrogen ions contributes to theconditioning process being accelerated and the electrochemical reactionsassociated with the conditioning process being of reduced durationcompared to previously-proposed devices. Not only is the time taken forthe conditioning process reduced, but also some of the non-uniformitiesassociated therewith are reduced. Thus, memory cells having a lowerstatistical spread of operating characteristics and being of increasedreliability than previously-proposed memory cells may be produced.Hence, the electrical resistance of a memory cell 10 according to anembodiment of the present invention can be switched faster than is thecase for previously-proposed devices.

In an embodiment of the present invention, the hydrogen reservoirstructure 2 comprises a dielectric material, hereinafter referred to asa first dielectric material, which is selected to comprise one of: ozonetetraethoxysilane (TEOS), a transition-metal oxide, a metal hydroxideand a zeolite. In this regard and for the sake of example, thetransition-metal oxide may be hydrogen tungsten oxide (H-WO3), the metalhydroxide may be one of aluminium hydroxide (Al(OH)₃), strontiumhydroxide (Sr(OH)₂) and calcium hydroxide (Ca(OH)₂) and the zeolite maybe hydrogen silicon oxide (H₂Si₂O₅).

In an embodiment of the present invention, at least one of theelectrodes M1, M2 and/or the resistive structure 1 is at least partiallyembedded in the hydrogen reservoir structure 2. Thus, the area overwhich migration of the hydrogen from the hydrogen reservoir structure 2into the transition-metal oxide is increased. Hence, this feature maycontribute to further accelerating the conditioning process.Alternatively, the hydrogen reservoir structure 2 is provided integrallyto at least one of the electrodes M1, M2 rather than in the dielectricmaterial 5, that is typically formed around the memory cell 10 and whichis, for example, silicon dioxide. This may be done by using a materialcomprising hydrogen for the electrode M1, M2, for example, a metal thatstores hydrogen. It may also be done by incorporating particles of amaterial comprising hydrogen into the electrode M1, M2, for example, byembedding the particles in the electrode M1, M2. An embodiment of thepresent invention is, of course, not limited to providing theabove-described specific arrangements of the hydrogen reservoirstructure 2 in isolation and they may be provided in combination witheach other. In fact, the scope of the present invention extends to anyarrangement of the hydrogen reservoir structure 2 that accommodates themigration of hydrogen into the resistive structure 1.

As can be seen from FIG. 1A, the electrodes M1, M2 of the memory cell 10include a metallization layer 3 made of platinum, for example, forcoupling to the resistive structure 1. For improved contact with themetallization layer 3, an adhesion layer 4 may be coated on thesurface(s) of the electrodes M1, M2. In this regard, the metallizationlayer 3 may also be provided with the adhesion layer 4. The material ofthe adhesion layer 4 may be titanium (Ti), titanium nitride (TiN),tantamum (Ta) or tantamum nitride (TaN), for example. In order to aidthe migration of hydrogen from the hydrogen reservoir structure 2 intothe resistive structure 1, the electrodes M1, M2 comprise material thatis permeable to and/or absorb hydrogen.

In an embodiment of the present invention, the electrodes M1, M2 arealso taken to comprise at least an electrical connector, for example avia plug for connecting to a foremost layer of a CMOS layeredarrangement. This arrangement is shown in FIG. 1B where the electrode M2is depicted as the via plug.

FIG. 2A shows a memory cell 20 according to another embodiment of thepresent invention. In this case, the electrodes M1, M2 include acoupling layer 6, by way of which they are connected to the resistivestructure 1. The material of the coupling layer 6 is selected to absorband/or have an affinity for hydrogen. Thus, the probability of thehydrogen migrating from the hydrogen reservoir structure 2 being drawninto the resistive structure 1, where it contributes to accelerating theconditioning process, is increased. In this regard, it is preferablethat the material of the coupling layer 6 absorbs at least 0.1 weight %of hydrogen. By way of example, the material of the coupling layer 6comprises one of: palladium (Pd), iridium (Ir), rhodium (Rd), a hydrideand an alloy comprising hydrogen. In this regard and for the sake ofexample, the hydride may comprise one of yttrium hydride (YH₂) andlanthamum hydride (LaH₂) and the alloy may comprise one of magnesiumnickel hydride (Mg₂NiH₄) and lanthamum nickel hydride (LaNi₅H₆).

In an embodiment of the present invention where the hydrogen reservoirstructure 2 is provided integrally to at least one of the electrodes M1,M2, this may be done by incorporating particles comprising hydrogen inthe coupling layer 6. Where the material of the coupling layer 6 doesnot contain hydrogen, for example, where the coupling layer 6 comprisespalladium, iridium or rhodium, the coupling layer 6 may be annealed inan atmosphere comprising hydrogen, thereby to incorporate the hydrogenin the coupling layer 6.

FIG. 2B depicts an alternative arrangement of the embodiment shown inFIG. 2A in that the coupling layer 6 is only provided to one of theelectrodes M1, M2 and that the other of the two electrodes is a via plugfor connecting to a CMOS substrate.

The present invention also extends to a non-volatile memory devicecomprising at least one memory cell 10, 20 according to an embodiment ofthe present invention and as described above. Where the non-volatilememory device comprises an array of memory cells 10, 20 according to anembodiment of the present invention, the hydrogen reservoir structure 2is provided so as to be shared by adjacent memory cells 10, 20. Thiscould be implemented in the array by providing the hydrogen reservoirstructure 2 continuously between all of the memory cells 10, 20 or someadjacent memory cells 10, 20.

FIG. 3 shows experimental results pertaining to an embodiment of thepresent invention. The results depict the monitoring of a conditioningprocess by a plot of current flowing in the conductive region of amemory cell as a function of time, where both parameters were measuredin arbitrary units. In the experiment, a memory cell 10 as shown in FIG.1A, but without the hydrogen reservoir structure 2, was respectivelysubjected to no hydrogen as shown in graph A, water in gaseous form asshown in graph B and hydrogen gas as shown in graph C. The dielectricmaterial of the resistive structure 1 was a transition-metal oxide inthe experiment, specifically, chromium-doped strontium titanate. Thegases were introduced in a vacuum chamber in which the memory cell 10had been placed. An electrical field of 1.10⁶ V/m was applied betweenthe electrodes M1, M2 so as to initiate the conditioning process.

As can be seen from graph A in FIG. 3, the magnitude of the current flowinitiated in the conductive region by the application of a voltagebetween the electrodes M1, M2 is approximately 2E-7 and does notincrease over the time range that the current is measured. This resultdepicts the conditioning process in previously-proposed devices.Specifically, the result shows that no conditioning process takes placein the measurement timeframe of the described-experiment inpreviously-proposed devices.

With reference being made to graph B in FIG. 3, it can be seen that byintroducing water in gaseous form in the vicinity of the memory cell 10,the conditioning process was initiated at a higher magnitude of current,i.e. approximately 1E-5, than was the case in graph A. Furthermore, at atime measure of 20 arbitrary units, the magnitude of the current startedincreasing and reached a value of 1 by a time measurement of 700arbitrary units.

With reference now being made to graph C in FIG. 3, it can be seen thatby introducing hydrogen in the vicinity of the memory cell 10, theconditioning process was initiated at a higher magnitude of current,i.e. approximately 3E-4, than was the case for either of graphs A or B.Furthermore, and advantageously, the current had reached a value of 1 bya time measurement of 50 arbitrary units.

The results shown in FIG. 3 support the fact that the presence ofhydrogen ions in the transition-metal oxide layer accelerates theconditioning process initiated therein. This formed the basis forintroducing the hydrogen reservoir structure 2 in an embodiment of thepresent invention.

Reference is now made to FIG. 4, which schematically illustrates amethod according to an embodiment of the present invention. A methodaccording to an embodiment of the present invention is started by, in astep S1, forming a resistive structure 1. The material is of theresistive structure 1 is selected to be transition-metal oxide. In astep S2, at least two electrodes M1, M2 are formed so as to be coupledto the resistive structure 1.

In a step S3, a hydrogen reservoir structure 2 is formed, marking theend of the process. A method according to an embodiment of the presentinvention is not limited to being performed once, i.e. after thecompletion of step S3, the process may loop back to the start of themethod and steps S1 to S3 may be performed iteratively, thereby toproduce multiple layers of memory cells according to an embodiment ofthe present invention. Any of the steps S1 to S3 can be performed inparallel or without maintaining a strict order of sequence. Any suitabletechnique known to a skilled person can be used for any one of steps S1to S3. The method described with reference to FIG. 4 can be supplementedwith further steps corresponding to features in a memory cell accordingto an embodiment of the present invention as described above.

Whilst an embodiment of the present invention has been described withreference to a stacked arrangement of the resistive structure 2 and theat least two electrodes, M1, M2, the present invention is not limitedthereto and any suitable arrangement is taken to fall within the scopeof the present invention, for example, where the resistive structure 2and the electrodes M1, M2 are arranged parallel to the x-plane.

An embodiment of the present invention is advantageously applicable tomaterials with more than two persistent resistance states.

The present invention has been described above purely by way of exampleand modifications of detail can be made within the scope of theinvention.

Each feature disclosed in the description and, where appropriate, theclaims and drawings may be provided independently or in any appropriatecombination.

1. A memory cell comprising: a resistive structure; at least twoelectrodes coupled to the resistive structure, and at least one hydrogenreservoir structure, wherein the application of an electrical signal toone of the at least two electrodes causes the electrical resistance ofthe resistive structure to be modified by altering a hydrogen-ionconcentration in the resistive structure.
 2. A memory cell as claimed inclaim 1, wherein at least one of the electrodes and/or the resistivestructure is at least partially embedded in the hydrogen reservoirstructure.
 3. A memory cell as claimed in claim 1, wherein the hydrogenreservoir structure is provided integrally to at least one of theelectrodes.
 4. A memory cell as claimed in claim 1, wherein at least oneof the electrodes is permeable to hydrogen.
 5. A memory cell as claimedin claim 1, wherein at least one of the electrodes comprises a couplinglayer for coupling the electrode to the resistive structure, thecoupling layer comprising a material that absorbs at least 0.1weight-percent of hydrogen.
 6. A memory cell as claimed in claim 5,wherein the coupling layer comprises one of: palladium, iridium,rhodium, a hydride and an alloy comprising hydrogen.
 7. A memory cell asclaimed in claim 1, wherein the hydrogen reservoir structure comprises afirst dielectric material.
 8. A memory cell as claimed in claim 7,wherein the first dielectric material comprises one of: ozonetetraethoxysilane (TEOS), a transition-metal oxide, a metal hydroxideand a zeolite.
 9. A memory cell as claimed in claim 1, wherein theresistive structure comprises a transition-metal oxide.
 10. Anon-volatile memory device comprising at least one memory cell asclaimed in claim
 1. 11. A non-volatile memory device comprising at leasttwo memory cells as claimed in claim 1, wherein the hydrogen reservoirstructure is provided so as to be shared by the at least two memorycells.
 12. A method for fabricating a memory cell comprising the stepsof: providing a resistive structure (S1); coupling at least twoelectrodes to the resistive structure (S2), and providing at least onehydrogen reservoir structure (S3).
 13. A method for fabricating a memorycell as claimed in claim 12 wherein, in the step of providing the atleast one hydrogen reservoir structure, at least one of the electrodesand/or the resistive structure is at least partially embedded in thehydrogen reservoir structure.
 14. A method for fabricating a memory cellas claimed in claim 12 wherein, in the step of providing the at leastone hydrogen reservoir structure, the hydrogen reservoir structure isprovided integrally to at least one of the electrodes.
 15. A method forfabricating a memory cell as claimed in claim 12, wherein at least oneof the electrodes is selected to be permeable to hydrogen.
 16. A methodfor fabricating a memory cell as claimed in claim 12, wherein at leastone of the electrodes comprises a coupling layer for coupling theelectrode to the resistive structure, the coupling layer comprising amaterial that absorbs at least 0.1 weight-percent of hydrogen.
 17. Amethod for fabricating a memory cell as claimed in claim 16, wherein thecoupling layer is selected to comprise one of: palladium, iridium,rhodium, a hydride and an alloy comprising hydrogen.
 18. A method forfabricating a memory cell as claimed in claim 12, wherein the hydrogenreservoir structure is selected to comprise a first dielectric material.19. A method for fabricating a memory cell as claimed in claim 18,wherein the first dielectric material is selected to comprise one of:ozone tetraethoxysilane (TEOS), a transition-metal oxide, a metalhydroxide and a zeolite.
 20. A method for fabricating a memory cell asclaimed in claim 12, wherein the resistive structure is selected tocomprise a transition-metal oxide.